JPH02149942A - Optical head - Google Patents

Abstract

PURPOSE:To generate multiple laser beams by means of a simple optical system without being affected by outside disturbing light by modulating laser beams emitted from a laser beam source LD at a constant repeating period and dealing the obtained signal with the constant repeating period as a carrier. CONSTITUTION:A light beam modulation source 11 modulates the laser beams emitted from the laser beam source LD, and photodetectors PD1 and SD2 convert reflecting laser beams from an optical disk into electric signals and modulation signals are turned as the carrier in a tuner 12. Information is obtained by demodulating the tuning signal in a demodulator 13. On the other hand, information can separately be obtained by converging two kinds of laser beams emitted from the laser beam source LD in the different tracks of the optical disk. Thus an optical head where the influence of the outside disturbing light beams and noise are removed can be realized.

Description

[Detailed Description of the Invention] [Summary] Regarding an optical head of an optical disk device used as a file device in a computer system, an optical head that is unaffected by ambient light and capable of producing multiple beams with a simple optical system is provided. With the aim of realizing this, the light beam used is modulated with a light beam modulation power source.
After light is received by a light receiving element, this modulated signal is tuned as a carrier wave by a tuner, and the signal from the tuner is demodulated by a demodulator, thereby obtaining information.

[Industrial application field]

The present invention relates to an optical head of an optical disk device used as a file device in a computer system.

[Conventional technology]

FIG. 13 is a block diagram illustrating the configuration of a conventional optical head.

The optical head 1 includes a laser light source LD, a collimator lens 3,
Anamorphic prism 4, beam splitter 5a,
Polarizing beam splitter 5b, objective lens 6, λ/2 plate 7
, lenses 8a, 8b, photodetector PD1. It consists of PD2, and the outline of its operation is as follows.

Receiving laser oscillation power from the light beam power source 9,
The laser light source LD oscillates, and the linearly polarized laser light emitted from the laser light 'aLD is converted into parallel light by the collimator lens 3.

Thereafter, it is converted into a perfectly circular laser beam by the anamorphic prism 4, transmitted through the beam splitter 5a, and focused onto the optical disk 2 by the objective lens 6.

The plane of polarization of the reflected light from the optical disc 2 is rotated according to the recorded information on the optical disc 2, and after being converted into parallel light by the objective lens 6, it is passed through the λ/2 plate 7 by the beam splitter 5a.
reflected to the side, polarizing beam splitter 5b, lens 8a
, 8b enters a differential optical system consisting of photodetectors PDI and PO2.

The reflected light incident on the differential optical system has its polarization plane rotated by 45 degrees by the λ/2 plate 7, is separated via the polarizing beam splitter 5b, and a portion is focused on the photodetector PDI by the lens 8a. The other part is focused on the photodetector PD2 by the lens 8b.

The laser light focused on the photodetector is converted into an electrical signal by the photodetectors PDi and PO2, and this signal is amplified by the differential amplifier 10 and becomes an output signal of the optical head.

FIG. 14 is a diagram illustrating the current-time characteristics of the light beam power source 9. The laser light source LD has a constant input current of 10
However, the laser oscillation becomes unstable due to the return light from the optical system of the optical head l, so a high frequency superimposed current iRF is superimposed on the input current 10 to stop the laser oscillation from the laser light source LD. It is stabilizing.

FIG. 15 shows the output signals of photodetectors PDI and PO2. The output voltage e0 of the output signal changes continuously as shown in the figure, according to the information recorded on the optical disc.

[Problem to be solved by the invention]

The optical head configured as described above has the following technical problems.

(2) The output voltage e0 obtained from the conventional optical head 1 is obtained by detecting the rotation of the plane of polarization of the reflected light from the optical disk 2 using the differential output of the differential optical system.

Therefore, if the disturbance light enters the optical head or enters the optical system before the polarizing beam splitter 5b, the effect of the disturbance light can be eliminated by the differential detection method described above (however, the influence of the disturbance light may be (However, disturbances in the polarization state cannot be removed.) If disturbance light enters the optical system after the polarizing beam splitter 5b, it will appear as it is in the output of the differential optical system, becoming noise and causing damage to the optical disk device. This may cause malfunction.

■If you are trying to erase, write, or read information on an optical disk at the same time, for example, if you are trying to erase and write or write and read information at the same time, use multiple laser beams in the optical head. This is done by setting the focal position of each beam at a different position on the optical disk.

Therefore, on the laser beam emission side, the laser light source LD
The focusing position is set by utilizing the difference in the light emitting points, and the position of the photodetector is also changed. Laser beams of different wavelengths are also used to separate multiple beams.

However, when using the difference in the light emitting point of the laser light source LD, it is very difficult to set the position of the photodetector, and high precision is required for the setting accuracy. Furthermore, when using laser beams with different wavelengths, it is very difficult to match the focal lengths of the laser beams with different wavelengths using the same objective lens.
High precision lens design is required. Therefore, the number of parts used increases and the weight also increases.

A technical object of the present invention is to solve such problems in optical heads and to realize an optical head that is not affected by ambient light and that can produce multiple beams using a simple optical system.

[Means to solve the problem]

FIG. 1(a) is a block diagram illustrating the basic principle of the present invention.

Here, the optical system of the optical head l has the same configuration as the conventional one, including a laser light source LD, a collimator lens 3, an anamorphic prism 4, a beam splitter 5a, a polarizing beam splitter 5b, an objective lens 6, and a λ/2 plate 7. , lens 8a
, 8b, and photodetectors PDI and PD2.

The present invention modulates the laser beam emitted from the laser light source LD by a light beam modulation power source (11), and converts the reflected laser beam from the optical disk into an electric signal by the photodetectors PDI and PD2, and then modulates the laser beam. Information is obtained by performing tuning with a tuner (12) using the signal as a carrier wave, and demodulating the tuned signal from the tuner (12) with a demodulator (13).

[Effect]

FIGS. 1(b), (c), (d), and (e) are diagrams explaining the present invention in detail.

The present invention is characterized in that by modulating the laser beam emitted from the laser light source LD with a constant repetition period, a signal of the repetition period obtained from the photodetector is treated as a carrier wave.

The signal obtained from the photodetector has an output spectrum as shown in (b). (b) In the diagram, the horizontal axis shows the frequency,
The vertical axis represents the photodetector output.

The spectrum of the angular frequency ω1 is the spectrum of the carrier wave, and the spectrum indicated by ω7o is the noise spectrum due to disturbance light. On the other hand, the tuner 12 has a selection characteristic BW, and outputs the photodetector output signal of the spectrum group (ω1 and ω7°) to the tuner 12.
2, the spectrum with the angular frequency ω1 is allowed to pass through as it is, while the noise spectrum indicated by ω7° is blocked from passing, making it possible to obtain an output spectrum from which the noise spectrum has been removed.

(c) is a diagram for explaining the output spectrum of the tuner 12, in which the horizontal axis represents the frequency and the vertical axis represents the output of the tuner 12. It is shown that only the carrier spectrum of the angular frequency ω1 from which the noise spectrum has been removed is obtained by the tuner.

(d) The figure shows the output signal of the tuner 12, with the horizontal axis representing time and
The vertical axis is expressed as amplitude.

When the laser beam is reflected by the optical disk, the plane of polarization rotates according to the recorded information on the optical disk, and the polarization beam splitter 5b converts the rotation of the plane of polarization into the intensity of the laser beam. The amplitude of the obtained carrier wave changes as shown in Figure (d). That is, the information on the optical disk appears in the envelope of the carrier wave (ωI) 21. Therefore, if the envelope is extracted by the demodulator 13, the recorded information can be obtained as a demodulated output (Sie+).

Here, since the recorded information on the optical disk is obtained from the envelope of the carrier wave, theoretically, the frequency of the carrier wave, that is, the modulation frequency of the light beam modulation power source 11 is 2 times higher than the maximum repetition frequency of the recording signal on the optical disk. It is necessary to have more than twice as many errors, but there are no read or write errors (
In order to do so, it actually needs to be four times or more.

In addition, the spectrum of angular frequency ω2 is shown in figure (b), but this is because in figure (a), a laser light source LD that emits two laser beams is used, and each laser beam has an angular frequency ω1, A case in which modulation of angular frequency ω2 is applied will be explained.

In this case, by using a tuner consisting of selection characteristics 8W+ and BWt, the carrier spectra ω1 and ω2 are
can be obtained separately. In the figure (c), this is the spectrum of the angular frequency ω2 indicated by the dotted line.

Therefore, by independently demodulating these carrier waves, independent demodulated outputs can be obtained. (e)
The figure is a diagram explaining this, and the demodulated output (Stgg) 24 is obtained from the envelope of the carrier wave (ω2) 22. That is, from the envelope of the carrier wave (ω2) 21 shown in figure (d), the demodulated output (Si, 1) 23 is obtained, and the carrier wave (ω2) shown in figure (e) is obtained.
A demodulated output (S89□) 24 is obtained from the envelope of ω2) 22.

Therefore, by focusing the two laser beams emitted from the laser light source LD onto separate tracks on the optical disk, even if the optical system on the light receiving side is for the l beam,
The information signal for each track can be obtained separately.

〔Example〕

Embodiment ■ Figures 2 and 3 are diagrams explaining an example in which amplitude modulation is applied to the laser light source LD of the optical head explained in Figure 1. Current Two-Hour Characteristics FIG. 3 is a diagram illustrating temporal changes in the output voltage of the photodetector.

In FIG. 1, the input current of the light beam modulation power source 11 is
As shown in Fig. 2, the input current 11 is a square wave with a constant period.
By setting, the laser light output emitted from the laser light source LD is also the input current i of the square wave.

undergoes similar modulation. At this time, the change in laser light output is used as a carrier wave. Furthermore, the waveform of the modulated wave of the light beam modulation power supply shown in FIG. 2 is not limited to a square wave, and may be a sine wave or the like.

When the laser beam is reflected by the optical disk, the plane of polarization rotates according to the information recorded on the optical disk, and the rotation of the plane of polarization is converted into the intensity of the laser beam by the polarizing beam splitter 5b. Therefore, the intensity of the laser beam is obtained from the photodetector as a change in the amplitude of the carrier wave, and the output voltage e of the photodetector changes as shown in FIG.

Therefore, information on the optical disk can be obtained from the envelope 14 connecting the peak values of the output voltage e of the photodetector.

Example 2 FIG. 4 shows an example using frequency-modulated laser light. In this case, the laser light source LDI is one whose laser oscillation frequency can be varied, and the laser oscillation power is supplied from the light beam power supply 15, and the modulation current for changing the laser oscillation frequency is supplied from the modulation power supply 16. receive. That is, the laser light source LD is activated by the light beam power source 15.
I continuously oscillates, and the modulation power supply 16 causes the laser light source L to
The oscillation frequency of DI is modulated.

FIG. 5 is a diagram illustrating the current-time characteristics of the modulation power source and the angular frequency of the laser beam that changes correspondingly.

In the figure, by setting the modulation current of the modulation power source 16 to a modulation current i of a square wave with a constant period and an angular frequency ω, the frequency of the laser light emitted from the laser light source LDI also becomes
It undergoes modulation similar to the square wave modulation current i. That is, the laser light source LDI continues to oscillate, but its oscillation frequency changes to ω□ and ω,2 in accordance with the modulation current i.

On the other hand, the photodetector is provided with wavelength dependence, and changes in the oscillation frequency of the laser beam are converted into changes in the amplitude of the output of the photodetector. FIG. 6 is a diagram illustrating filter characteristics for imparting wavelength dependence to a photodetector. In the figure,
A filter F with a cut-off angular frequency ω is connected to a photodetector PDI, P
By providing it on the light receiving surface of D2, the laser beam with an angular frequency ω, 2 passes through the filter F, and the photodetector PDI,
However, the laser beam with the angular frequency ωrl cannot pass through the filter F and cannot reach the photodetectors PDI and PO2. Therefore, the frequency-modulated laser beam appears as the output of the photodetector only when the angular frequency of the laser beam is ω1□, so the output of the photodetectors PDI and PD2 has the same modulation current i. Since a modulated wave of angular frequency can be obtained, this is selected as a carrier wave by the tuner 12.

In addition, since the intensity of the laser light reaching the photodetectors PDI and PD2 changes according to the information recorded on the optical disk by the polarizing beam splitter 5b, the envelope of the carrier wave obtained by the tuner 12 is changed by the demodulator 13. By taking it out, the recorded information on the optical disc can be obtained. In this modulation method, the angular frequency of the laser light from the laser light source LD1 changes due to modulation, but the laser light output does not change, so it is possible to irradiate the optical disk with laser light having a high average power.

Therefore, this is an advantageous modulation method when a large amount of laser light output is required.

Embodiment 2 FIG. 7 is a diagram illustrating an optical helad 1 using two laser beams. The operation in this case has been explained in [Operation] above, so the configuration will be explained here.

A laser light source LD2 that emits two laser beams is used as the laser light source of the optical head 1, and each laser beam is provided with a light beam #1 modulation power source (ω1) 17 and a light beam 112 modulation power source (ωt) 18. Amplitude modulation is applied at angular frequencies ω and ω2, respectively. In this case, the frequencies of each laser beam may be the same, but of course may be different.

FIG. 8 is a diagram illustrating the power supply current characteristics of the light beam 11 modulated power source (ω1), in which modulation is applied by the input current 11 of the angular frequency ω1. Figure 9 shows the light beam #2 modulated power source (ω2
) is a diagram illustrating the power supply current characteristics of ω2, in which modulation is applied by input current 12 of angular frequency ω2.

On the other hand, the tuner 12a separately selects the carrier wave with the angular frequency ω and the carrier wave with the angular frequency ω2, and the demodulator 13a extracts the envelope of each carrier wave separately, thereby recording on the optical disk. Information can be retrieved separately.

Example ■ Example ■ above is a case in which two beams of laser light are subjected to amplitude modulation with different angular frequencies, but the laser oscillation frequency of each laser beam is subjected to frequency modulation with different angular frequencies. Good too. However, in this case, as explained in Example (2), it is necessary to make the photodetectors PDI and PO2 wavelength dependent.

Example ■ Example ■ is a case in which two beams of laser light are subjected to amplitude modulation, each with a different angular frequency.However, even if the angular frequency is the same, amplitude modulation is applied to each beam with a modulated wave of a different phase. You can put it on. However, in this case, the output of the tuner 12a is
Since carrier waves with different phases and the same frequency are synthesized and output as a single frequency carrier wave, the demodulator 13g uses the single frequency synthesized carrier wave and the carrier wave to modulate the laser light source LD2. By synchronizing the phase of the modulation signal, each carrier wave can be separated, and the recorded information on the optical disk can be separately extracted from the envelope of each separated carrier wave.

Example ■ Example ■ above is a case where two beams of laser light are frequency modulated with different angular frequencies, but even if the angular frequencies are the same, frequency modulation is performed with modulated waves of different phases. You can put it on. However, in this case, the output of the tuner 12a is output as a carrier wave of a single frequency, which is a combination of carrier waves of different phases and the same frequency. and,
By synchronizing the phase of the modulation signal used for modulating the laser light source LD2, each carrier wave is separated, and the recorded information on the optical disk is separately extracted from the envelope of each separated carrier wave. Can be done.

Example ■ (A) The above Example ■ is a case in which two beams of laser light are subjected to amplitude modulation with different angular frequencies. Frequency modulation of an angular frequency different from the frequency may be applied.

However, in this case, it is necessary to use a filter whose cut-off angular frequency allows the angular frequency of the amplitude-modulated laser light to pass through in order to make the photodetector wavelength dependent.

(b) In an optical head using a laser beam laser with 3 beams or more, by combining the modulation and demodulation methods described in a) to d) below, the optical system can be kept the same as before.
A demodulated output signal corresponding to the beam is obtained.

a) Using modulated waves of different angular frequencies in amplitude modulation.

b) Using modulated waves of different angular frequencies in frequency modulation.

C) Using modulated waves with the same angular frequency but different phases in amplitude modulation.

d) Using modulated waves of the same angular frequency but different phases in frequency modulation.

Embodiment 2 FIG. 10 shows an embodiment in which the signal processing system is changed by changing the modulation frequency of the light beam.

The laser light source LD of the optical head 1 has one beam, but by changing the modulation angular frequency of the light beam modulation power source 19 to ω1 and ω2, carrier waves of each angular frequency can be selected separately from the tuner 12a. , and separately demodulating the separated carrier waves in a demodulator 13a, separate demodulator outputs II, 112 are obtained.

In this case, the modulation of the laser beam may be either amplitude modulation or frequency modulation. In addition, by keeping the modulation frequency constant and changing the phase of the modulated wave, Example ■ and Example ■
Separate demodulator outputs 111J2 can also be obtained, as explained in .

This embodiment is effective when picking up, for example, a video recording signal or an audio recording signal with one optical head. That is, since the recording signal on the optical disk demodulated by the demodulator undergoes different signal processing for the video recording signal and the audio recording signal, it is necessary to process them in separate signal processing systems. Therefore, at the demodulator output Ill, 112,
By connecting the video recording signal processing system and the audio recording signal processing system, respectively, and switching only the modulation angular frequency of the light beam modulation power source 19, the required recording signal can be obtained immediately.

Example ■ The above Example ■ uses a one-beam laser light source LD,
By changing the modulation frequency of the laser beam,
The signal processing system is changed, but in this example, 2
The signal processing system is changed by using the beam laser light source LD2 and changing the modulation frequency of each laser beam. FIG. 11 shows its configuration.

A light beam of $1 is applied to each laser beam of the laser light source LD2.
A modulated power supply 20 and a light beam $2 modulated power a! With X21,
Different modulations are applied, and different demodulator outputs III, 112 are obtained depending on the modulation frequency at this time.

That is, the light beam $1 modulated power source 20 and the light beam $2
By changing the modulation angular frequency to ω1 and ω2 for each of the modulated power sources of the modulated power source 21, a carrier wave of each angular frequency is separately selected and separated from the tuner 12a, and the separated carrier wave is By separately demodulating in the demodulator 13a, separate demodulator outputs Ill, 12 are obtained. Therefore, since a separate demodulator output ill, 112 is obtained for each light beam, there are four combinations of each light beam and the output.

FIG. 12 is a diagram illustrating the above combination.

If the modulation angular frequency of the light beam $1 is ω, the output Ill
is obtained, and if the modulation angular frequency is ω2, an output I2 is obtained. The same applies to the light beam $2.

Embodiment [Phase] The above embodiment (2) uses a two-beam laser light source LD2 and changes the signal processing system by changing the modulation frequency of each laser beam. In the optical head that uses a laser beam, by combining the modulation and demodulation methods described in a) to d) below, the signal processing system can be changed according to the beam while the optical system remains the same as before. be able to.

a) Using modulated waves of different angular frequencies in amplitude modulation.

b) Using modulated waves of different angular frequencies in frequency modulation.

C) Using modulated waves with the same angular frequency but different phases in amplitude modulation.

d) Using modulated waves of the same angular frequency but different phases in frequency modulation.

Example ■ In the above Examples ■ to [Phase], even if one of the light beams used is unmodulated as in the conventional case, the tuner 12a should be provided with a low-pass selection characteristic. Thus, information recorded on the optical disc can be obtained in the same way as in the past.

[Effects of the Invention] As described above, according to the present invention, the laser beam of the optical head is modulated and this modulated wave is selected as a carrier wave by a tuner, so that the optical head eliminates the influence of disturbance light and noise. can be realized.

Furthermore, since the information of a plurality of light beams can be separated, the optical system of the optical head can be used almost as is for one beam, and the number of parts and weight of the optical head can be reduced.

Furthermore, by changing the modulation frequency, an optical head whose signal processing system can be changed can be realized.

[Brief explanation of the drawing]

FIG. 1 is a diagram explaining the basic principle of the present invention, in which (a) is a block diagram of an optical head, (b) is an output spectrum of a photodetector, (c) is an output spectrum of a tuner, (
Figures d) and (e) show the output amplitude of the tuner, Figure 2 shows the current-time characteristics of the optical beam modulation power source, Figure 3 shows the temporal change in the output voltage of the photodetector, and Figure 4 shows the output voltage of the photodetector. , a block diagram of an embodiment of an optical head using a frequency-modulated laser beam, FIG. 5 is a diagram explaining the current-time characteristics of the modulation power supply and the angular frequency of the laser beam, and FIG. 6 is a diagram for explaining the optical detection Figure 7 is a block diagram illustrating an example of an optical head using two laser beams, and Figure 8 is a filter characteristic diagram for making the device wavelength dependent.
Figure 9 is a power supply current characteristic diagram of a modified power supply (ω,), Figure 9 is a power supply current characteristic diagram of a light beam I2 modulation power supply (0 m), and Figure 10 is a diagram of the signal processing system changed by changing the modulation frequency. FIG. 11 is a block diagram of an optical head embodiment configured to use two laser beams and change the signal processing system by switching the modulation frequency. FIG. 12 is a diagram explaining the combination of light beam and output, FIG. 13 is a block diagram explaining a conventional optical head, FIG. 14 is a current-time characteristic diagram of the light beam power source, and FIG.
The figure is a waveform diagram of the output signal voltage of the photodetector. In the figure, 1 is an optical head, 2 is an optical disk, 3 is a collimator lens, 4 is an anamorphic prism, 5a is a beam splitter, 5b is a polarizing beam splitter, 6 is an objective lens, 7 is a λ/2 plate, and 8a. 8b is a lens, 9 and 15 are light beam power supplies, 10 is a differential amplifier, 11 is a light beam modulation power supply, 12.12a is a tuner, 13.13a is a demodulator, 14 is an envelope, 16 is a modulation power supply, Reference numeral 17 indicates a light beam 11 modulation power supply, 18 a light beam #2 modulation power supply, 19 a light beam modulation power supply, 20 a light beam $1 modulation power supply, and 21 a light beam $2 modulation power supply. 20th Patent Applicant: Fujitsu Limited Sub-Agent Patent Attorney Yasushi Fukushima Figure 6 2 Beam, Mu, LD and the old color O construction Figure 72 Sample diagram: Denryokan Current - Time Moon Figure 14 Figure 160

Claims (1)

[Claims] 1. The light beam to be used is modulated by a light beam modulation power source (11), and after being received by a light receiving element, this modulated signal is tuned as a carrier wave by a tuner (12). An optical head characterized in that information is obtained by demodulating the signal from (12) with a demodulator (13). 2. The optical head according to claim 1, wherein the modulation applied to the light beam is amplitude modulation. 3. The optical head according to claim 1, wherein the modulation applied to the light beam is frequency modulation.